Measuring the interior features of a hole is critical in many industries. For example, manufacturers generally must use a variety of materials and fasteners to assemble a product. The fasteners hold together the materials, which may have assorted properties and dimensions. Typical product assemblies involve stacks of materials that may have different stack thicknesses. When the assemblers drill holes into and/or through the stacks of materials of different thickness, the result is a variety of hole lengths. The hole length or distance from the top edge of the hole to the bottom edge is commonly called “grip length.” A fastener must be carefully chosen to closely fit the hole's grip length in order to properly clamp the stack of materials together.
To determine the proper size fastener to select for connecting the materials of the assembly, one must know the grip length of the hole. The grip length of the fastener should match the grip length of the hole as closely as possible. The part of the fastener that defines the grip length may be different for different types of fasteners. In addition, the hole may be counter sunk and one must choose the proper size fastener to fit the counter sink of the hole. A counter sunk hole is formed such that the fastener fits completely within the hole and the fastener is flush with the outside of the assembly. Therefore, a counter sunk hole has a counter sink facing the outside of the assembly that is angled radially inward to receive the head of the fastener.
Selecting the correct size of fastener is essential in certain product assemblies because a fastener that is too long or too short could compromise the structural integrity and/or the safe and efficient operation of the product. In aerospace manufacturing, for example, if the product contains an incorrect size fastener, the product may fail to meet the structural requirements. If the fastener is too short or too long, it may not securely connect the materials, creating a risk that the materials will separate. Additionally, a fastener that is too long may cause excess weight and/or insufficient compression. The incorrect size fastener must be replaced, which causes the manufacturer to spend extra time and money for the assembly. Conversely, if the fastener is not replaced, the product may fail to meet the structural requirements and/or may not function as the manufacturers intended for the full life of the product.
Some assemblies contain thousands of holes with varying grip lengths for receiving fasteners. The grip length of each hole must be determined quickly, simply and accurately. Measuring the grip length of a hole may be difficult, particularly when measuring a “through hole,” which does not have a bottom. The conventional grip length measurement methods of calipers and depth gages are not suitable for through holes because they require a bottom surface from which to reference.
Measuring the grip length of a through hole is particularly difficult in instances in which the backside of the hole is inaccessible. This type of hole is termed a blind hole and requires a blind grip length measurement. One conventional tool for measuring blind grip length is a blind fastener grip gage. In general, this gage consists of a handle with a thin ruler piece extending from the handle. The ruler has a hook at the distal end and a slide on the ruler. To use this gage, one fully inserts the ruler piece into the hole and draws the gage back until the hook contacts the “blind” side of the hole. The operator then moves the slide toward the outside surface of the hole until it straddles both sides of the hole, which is an attempt to position the gage as perpendicular as possible to the outside surface and, thus, produce the most accurate a grip length measurement. Once these steps are completed, the operator removes the gage from the hole and reads where the slide coincides with the numbers on the ruler.
The problem with this conventional tool to measure blind grip length is that it does not provide quick, simple and accurate grip length measurements that are necessary in assemblies with thousands of holes. The measurements made with the conventional tool are not quick or simple because of the time and effort involved in manually inserting, moving, adjusting, sliding and reading the gage. The measurements made with the conventional tool are also not always accurate because the outside surface may not be level enough to ensure the gage is perpendicular. In addition, the processes of moving the slide and reading the ruler are subject to human error. This conventional tool also does not provide for automated data collection, which increases the likelihood of a mistake in recording the measurement and location of each hole.
Another significant drawback to the conventional grip length measurement tools described above is that the tool makes contact with the hole wall to measure the hole. In assemblies containing soft materials, the contact of the tool could potentially damage the hole. Damage to the hole may cause irregularities in the fastener connection within the hole. Furthermore, in many industries, drilling the holes creates a substance in and around the hole that may get on the tool when it contacts the hole and, over time, may impede proper use of the tool and cause inaccurate measurements.
Although a few conventional tools exist that do not contact the hole while measuring the characteristics of the hole, they nevertheless fail to provide a quick, simple and accurate method to measure the characteristics of holes. For example, one conventional method described by U.S. Pat. No. 5,895,927 is an electro-optic, non-contact probe that determines the interior physical characteristics of a tubular structure by using a disc of unfocused light to illuminate a cross-section of the interior surface. An image of the reflections from the illuminated cross-sections of the interior surface is then constructed by a photodetector array and evaluated. This structure is not as quick and simple of a solution to hole measurements as desired because it requires many precision parts and a complex construction to create the disc of light and construct the resulting image, which increases the likelihood of difficulties in using the probe. The precision parts and complex construction of the conventional noncontact probes also cause repair and/or maintenance to be time consuming.
For the reasons described above, the conventional tools to measure the dimensions of through holes do not provide the quick, simple and accurate method necessary to efficiently measure the dimensions of the thousands of holes common in many assemblies. Without an effective manner in which to measure the dimensions of the holes, particularly the grip length, it is impossible to assure the appropriate fasteners will be selected for the holes. The conventional tools described above require manual alignment and other user tasks before the tools can take measurements of the hole, which subjects the measurements to human error. In addition, the tools do not generally provide an effective means for automatic data collection. Some of the conventional tools described above also must contact the hole wall, creating a potential for damage to the hole. Unfortunately, even the conventional tools that do not contact the hole wall may be impractical because of the complexity of the construction. Thus, there exists a need in the industry for a tool that provides a quick, simple and accurate method for measuring the dimensions, particularly the grip length, of a hole and that does not contact the hole wall.
In addition to the problems described above regarding measuring the dimensions of through holes, particularly the grip length, it is also difficult to drill and/or measure the interface of two-step through holes using the conventional method. A two-step through hole is a hole that has different diameters at different axial locations or depths. In one common application, the hole is drilled and reamed through a stack of materials such that a hole of one diameter is reamed through a first material layer and a hole of another diameter is drilled through a second material layer. Typically, the hole in the material on the side of the stack that faces the inside of the assembly has a smaller diameter than the hole in the material on the side of the stack that faces the outside of the assembly. Two-step holes are typically utilized when fastening layers of composite materials, which are less resilient than, for example, metallic materials. A fastener inserted in a single diameter hole exerts a large amount of pressure on the hole walls and the pressure may cause the composite material around the hole to deteriorate or weaken over time. Two-step holes and corresponding fasteners reduce the risk of damage to composite materials by varying the diameter of the holes, which distributes the pressure exerted by the fastener upon the composite layers. The conventional method of drilling two-step holes in the aircraft industry is to first drill through the stack of material layers to create a hole of the smaller diameter, then separate the layers and use a reamer to ream a hole with a larger diameter in one of the layers. Both of the layers must be cleaned before reforming the stack to ensure no fragments of the material are caught between the layers which could cause inaccurate alignment of the holes in the stack. The conventional method of drilling and reaming a two-step hole is prone to human error and time consuming because of the numerous steps involved.
Even in instances in which a hole having a constant diameter is to be drilled through a stack of dissimilar materials, it may be desirable to determine the location of the interface between different material layers. For example, depending upon the types of materials, different drill bits may be required to drill through the different material layers. For the reasons described above in conjunction with drilling two-step holes, however, it is generally difficult to determine the locations of the interface between the material layers with the desired accuracy. Unfortunately, none of the conventional methods to measure grip length described above are designed to detect the interface between the materials of a stack so as to assist in drilling through the stack. Thus, there exists a need in the industry for a tool that not only measures the dimensions of through holes, particularly the grip length, but also automatically detects a change in the material composition and, thus, the interface between the materials. A tool that accurately and automatically detects the interface between the materials would greatly decrease the amount of time involved in drilling and reaming two-step holes and increase the precision of the resulting holes.